Individual graphene sheets have attracted significant attention since their isolation (Novoselov et al., Science 306, 666 (2004)), due to the multitude of graphene's exceptional properties (Gomez-Navarro et al., Nano Letters 7, 3499 (2007); Geim et al., Nature Materials 6, 183 (2007); Lee et al., Science 321, 385 (2008); Miller et al., Science 329, 1637 (2010); Novoselov et al., Science 315, 1379 (2007); Schedin et al., Nature Materials 6, 652 (2007); Zhu et al., Adv Mater 22, 3906 (2010); Chen et al., Nat Nano 3, 206 (2008).
Some of the properties observed in this nanoscale, two-dimensional (2D) form of graphitic carbon include room-temperature electrical conductivities up to ˜106 S/cm and Young's moduli up to ˜106 MPa, which are among the highest reported for any material. In an effort to realize the properties of individual graphene sheets on the macro-scale, several groups have recently developed three dimensional (3D) graphene assemblies with many promising characteristics.
Such assemblies are comprised of randomly interconnected graphene sheets with a large degree of porosity (>90%), which is believed to be necessary to minimize restacking of graphene sheets. These low-density nanoporous graphene structures exhibit electrical conductivities and Young's moduli as many as 10 orders of magnitude lower than those observed for individual graphene sheets, which is not surprising given their high porosity. It is a direct consequence of superlinear dependences of electrical and mechanical properties on the monolith density for porous materials.
Macro-scale 3D graphene-based materials exhibiting the exceptional properties, including combinations of properties, of graphene sheets are still a challenge. For example, electrical conductivity might be too low.